This was discussed this season by some of the LRI's R66 is very clear.
Quote:
R66
F. Pressure transducers, pressure gauges, passive flow control valves (specifically “needle valves”), manifolds, and
connecting fittings,
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Q227 and R78 outlaws using additional vent plugs to stop flow to a system. Since all vent plugs must release pressure to the system in a reasonable time.
Check valves are also illegal.
Q134 Does allow for pluming a solenoid's outputs into a another's input, but warns that the vent plug must always be able to dump all air. I'm not thinking of a simple way to do this. It might be possible for a single solenoid to switch states when it loses pilot pressure and vent the upstream air regardless of state but I'm not sure about that and would need to test it.
Using a needle valve to stop flow seems legal to me at the moment, and I would pass it at an event I was inspecting right now, any other type of valve would be illegal. That could easily change.